Continental shelf systems are areas formerly above sea level but now submerged. Such regions may have been periodically submerged with variations in sea levels over geological time. However, the continental shelf system off the US North East has come up with a big surprise – it harbours a lake underneath the sea bed – go to https://tallbloke.wordpress.com/2019/06/23/scientists-map-huge-undersea-… … the lake is actually very deep down below the sea bed and is described as an aquifer (a source of water from an ancient era). Normally, taking water from aquifers is frowned upon but the impetus of this piece seems to be it is okay to tap it as a source of fresh water for human use – but that is of course the journalist's take. It goes on to say such aquifers may lie offshore water starved countries in other parts of the world. The aquifer, a giant lake of water at least 50 miles in length, is said to be trapped in porous sediments. What does that mean? Are they referring to chalk which is porous and which allows water to sink down to other geological layers such as clays – where it tends then to move horizontally and feed rivers and streams. Chalk aquifers are exploited by water extraction companies which is perhaps why they think it okay to pipe water from this one. However, this is described as an underground lake which doesn't really fit into the usual chalk aquifer category. It stretches from Massachusetts to New Jersy and back in the 1970s oil drilling in the area (on the continental shelf system) occasionally hit water – hence the investigation (50 years later). What they have done is establish the dimensions of the aquifer and its reality.
Apparently, the aquifer is being fed via subterranean sources by modern run off from the land but the researchers are thinking in terms of it is a relic of the Late Glacial Maximum.